1. Related Applications
This application claims the priority of Japanese Patent Application No. 5-261903 filed on Sep. 24, 1993, which is incorporated herein by reference.
2. Field of the Invention
This invention relates to a method of manufacturing a light emitting device, and more precisely to a method of manufacturing a light emitting device wherein deterioration of brightness is effectively prevented by forming a protective layer on the AlGaAs chip surface.
3. Prior Art
A light emitting device generally comprises an n-electrode and a p-electrode provided on a compound semiconductor chip which has a pn junction, and the chip is molded entirely with epoxy resin. FIG. 4 shows an example of the structure of a conventional light emitting device. In this light emitting device, an AlGaAs chip 10 which comprises an n-type layer 11 and a p-type layer 12 has an n-electrode 13 on the n-type layer side and a p-electrode 14 on the p-type layer side, and the chip 10 is fixed to a base 16 by silver paste and such. The chip 10 is sealed entirely with epoxy resin mold 15.
In general, epoxy resin is hygroscopic and is poor in an effect of moisture prevention. Therefore, under high humidity, moisture in the air penetrates into the epoxy resin mold 15 and reacts with AlGaAs, particularly when an electric current flows in the light emitting device during operation, changing in quality of the surface. As shown in FIG. 5, an opaque altered layer 17 is generated on the top surface and the side surfaces of the n-type layer 11. When this opaque altered layer 17 is generated, the light extraction efficiency decreases and hence the brightness of the light emitting device decreases.
Because of this, a method has been proposed in which a translucent protective layer 18 primarily composed of aluminum oxide is formed on the top surface and the side surfaces of the AlGaAs chip 10 (the 42nd Japan Society of Applied Physics and Related Societies, pp 600, 9a-D-3, 1981, and the 44th Japan Society of Applied Physics and Related Societies, pp 485, 28a-H-3, 1983).
This method utilizes the fact that an aqueous solution containing ammonia and hydrogen peroxide reacts with AlGaAs mixed crystals to form an aluminum oxide coating layer. That is, the AlGaAs chip 10 is immersed in said ammonia-hydrogen peroxide aqueous solution to form a protective layer 18 primarily composed of aluminum oxide on the light extracting surface (hereafter referred to as the "upper surface") and the side surfaces so that the AlGaAs chip 10 is protected from the penetrating moisture. The thickness of this protective layer 18 is preferably 100 nm or more.
Also, Japanese unexamined patent publication (Tokkai) Hei 4-216683 proposes a method in which an AlGaAs chip is immersed in an aqueous solution containing 0.3-0.6 wt. % of ammonia and 25-35 wt. % of hydrogen peroxide to form a protective layer on said chip surface.
In the methods in which the chip is immersed in an ammonia-hydrogen peroxide aqueous solution, such as described above, the upper surface (the surface on the side of the n-type layer 11 in FIG. 4) or the surface which is to be fixed to a base (the surface on the side of the p-type layer 12 in FIG. 4, hereafter referred to as the "lower surface") is adhered to an adhesive sheet, and the AlGaAs chip 10, together with the adhesive sheet, is immersed in said ammonia-hydrogen peroxide aqueous solution. It is particularly preferable to perform the immersion treatment with the chip's lower surface adhered to the adhesive sheet. This is because the protective layer 18 is not formed or has difficulty forming on the surface which is adhered to the adhesive sheet, and the chip's lower surface does not need the protective layer 18 since it will be fixed to the base 16.
With such methods as described above, however, it was not possible to form protective layers with a uniform thickness. There was a significant difference between the generation rates of the protective layers on the side of the n-type layer 11 and on the side of the p-type layer 12 of the AlGaAs chip 10. That is, as shown in FIG. 6, a thick protective layer 18 with a thickness of 100 nm or more was formed on the side of the n-type layer 11, whereas the protective layer 18 was thinner on the side of the p-type layer 12, and it was impossible to form a stable protective layer 18 with a thickness over 100 nm on this side. Because of this, under high humidity, the upper surface and the side surfaces of the AlGaAs chip 10 were changed in quality from the side of the p-type layer 12, leading to a decrease in brightness and hence a decrease in the reliability of the light emitting device.
Even when the duration time of the immersion of the chip 10 in the ammonia-hydrogen peroxide aqueous solution was increased so as to thicken the protective layer 18 on the side of the p-type layer 12, it was still difficult to thicken only the protective layer 18 on the side of the p-type layer 12 and the thickness difference compared with the side of the n-type layer 11 became even greater. An effort to increase the thickness of the protective layer 18 on the side of the p-type layer 12 resulted in a protective layer 18 too thick on the side of the n-type layer before the thickness of the protective layer 18 on the side of the p-type layer 12 reached 100 nm, causing cracks and thus being impossible to formate a good protective layer throughout the entire chip surface.
Further, according to the methods, the AlGaAs chip 10 is immersed in an ammonia-hydrogen peroxide aqueous solution, and particularly the chip 10 is immersed in an ammonia-hydrogen peroxide aqueous solution with the lower chip surface (the surface which is to be fixed to the base, i.e. the surface on the side of the p-type layer 12 in FIG. 4) adhered to an adhesive sheet. There was a problem in that since the exchangeability with the aqueous solution became poorer near the adhesive sheet, the generation rate of the protective layer 18 on the side of the p-type layer 12 became smaller relative to the rate on the side of the n-type layer 11, hence the protective layer 18 on the side of the p-type layer 12 became even thinner.